Tm01 mode rf pulse generator



June 11, 1968 P. P. KEENAN 3,383,287

TMOI MODE RF PULSE GENERATOR Filed Jan. 4, 1965 2 sheets-sheet 1 4 4 291 l ONE-HALF I WAVE LENGTH I I PosmoN E I l I o U L J\ 2 L... 1|\,4 Vo

, (RF. INPUT) (RF. |NPUT) I9 22(+ o c. SUPPLY) 20/ ,9 o I4 I5 I /5 DIELECTRIC m` s-wAvEeuxDE @s wAvEGulDE DIELECTRIC 23 ENvELoPE F I Q 2 (-uc. SUPPLY) f-M 28 30 32 33 34 z INVENTOR. PETER F? KEENAN June 11, 1968 P. P. KEI-:NAN

TMOI MODE RF PULSE GENERATOR 2 Sheets-Sheet. 2

Filed Jan.

PLASMA DISCHARGE) 24 FIG-8 FIG-7 L INVENTOR. PETER P. KEENAN Agent United States Patent O 3,388,287 TMm MDE RF PULSE GENERATOR Peter I. Keenan, Van Nuys, Calif., assigner to Lockheed Aircraft Corporation, Burbank, Calif. Filed Jan. 4, 1965, Ser. No. 422,958 13 Claims. (Cl. 315-39) ABSTRACT F THE DISCLOSURE An extremely high-frequency wave generator is described which is capable of producing nanosecond pulses of RF energy at multi-kilowatt levels. The wavelengths of the generated waves may range from microwaves to sub-millimeter waves. Adjacent half-wave segments of circular TM01 Inode waveguide are supplied with charges of opposing polarity and thereafter coupled into a continuous series arrangement by means of electronic switches, thereby resulting in the propagation of an RF pulse in the TMm mode.

This invention relates to apparatus for generating extremely high-frequency RF energy, at very high power levels, and for extremely short pulse durations.

In one embodiment of this invention, a multi-section waveguide is periodically coupled into a continuous series arrangement by means of an electronic switch, thereby causing the self-generation of plasma waveguide phenomena which result in the propagation of RF energy.

The principle of operation can best be understood by considering the instantaneous condition of a voltage wave being propagated on a transmission line. In such an instance successive half-wave sections of the line will be oppositeiy charged. This instantaneous voltage distribution can be electrostatically simulated by slicing or separating the transmission line into a plurality of halfwave segments, each of which is separated by an isolating gap, and alternately charging successive segments to have opposite charges. However, such an electrostatic voltage distribution will not propagate, or radiate, electromagnetic energy since no current flow is involved. However, if all of the charged sections were to be simultaneously connected in series, current would flow and the wave formation would propagate down the transmission line as a true electromagnetic wave.

It has been proposed heretofore to apply this broad concept to an electrostatically charged artificial transmission line composed of lumped capacitances and inductances. However, the artificial line technique cannot be extended into the microwave region due to the finite size of the structural elements required. To generate microwave frequencies a waveguide technique is required. In accordance with the present invention the TMm mode waveguide is employed and is particularly suited for the intended function of the invention since the voltage distribution is constant over a given cross-section of the waveguide. That is, the instantaneous voltage field distribution for this waveguide mode can be simulated by simply alternately charging successive one-half wavelength sections of the 'IMol waveguide. Furthermore, an intense RF-induced plasma can be created in the region of such a waveguide by the so-called controlled waveguide mode technique, to be described in a subsequent part of this specification. The generation of an RF discharge plasma in the gaps between the alternately charged waveguide sections is used to effectively short across the gaps and initiate current How. As a result of this current iiow there will be generated an electromagnetic wave which is propagated down the waveguide. Microwaves and millimeter waves can thus be generated. It has, heretofore, been extremely difiicult to adapt magnetron and klystron techniques to the generation of millimeter and 3,388,287 Patented June 11, 1968 lCe sub-millirneter waves. Techniques presently used to generate these extremely short waves are expensive, low powered, and even inadequate since the tolerances are extremely critical. Prior to the present invention there has been no satisfactory technique for generating nanosecond pulses at multi-kilowatt levels at microwave or millimeter wavelengths.

The apparatus of the present invention is inherently simple and thus inexpensive to manufacture. Furthermore, high pulse powers, of the order of hundreds of kilowatts, and pulses of very short duration, of the order of nanoseconds, may be generated in accordance with the invention in both the microwave and millimeter wave region. The associated power supply requirements are substantially simpler than those employed in radar modulators and similar equipment.

It is therefore an object of the invention to provide novel means for the generation of electromagnetic space waves.

It is another object of the invention to provide novel and improved means for the generation of microwaves, millimeter waves, and sub-millimeter waves of radio frequency (RF) energy.

It is still another object of the invention to provide novel and improved means for the generation of extremely high frequency, RF energy at very high power levels.

Yet another object of the invention is to provide novel and improved apparatus for generating pulses of highfreqeuncy `RF energy having extremely short pulse durations.

It is another object of the invention to provide novel and improved apparatus, for generating ultra high frequency RF energy which does not require critical tolerances.

It is yet another object of the invention to provide novel and improved apparatus for generating extremely high frequency RF energy, which is simple in construction and inexpensive as compared with apparatus heretofore intended to accomplish generally similar purposes.

Still another object of the invention is to provide novel and improved apparatus employing controlled mode plasma techniques for the generation of RF energy.

These and other objects of the invention will become more readily understood upon consideration of the following specification taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a graphic representation of a voltage wave on a transmission line, wherein voltage amplitude is plotted versus propagation position, and is useful in the exposition of the invention;

FIGURE 2 is a somewhat diagrammatic perspective view, partially broken away, illustrating a preferred embodiment of a wave generator constructed in accordance with the invention;

FIGURE 3 is a diagrammatic illustration of a waveguide illustrating the field geometry therein;

FIGURE 4 is an elevation view of the sectional waveguide portion of the apparatus of FIGURE 2;

FIGURE 5 is similar to FIGURE 4 and illustrates the switching relating thereto;

FIGURE 6 is a perspective view, partially in section, illustrating a modification of the sectional waveguide portion of the invention;

FIGURE 7 is an enlarged detailed View of the choke section of the apparatus of FIGURE 6;

-FIGURE 8 is a fragmentary detailed view of a moditication of the apparatus of FIGURES 4 and 5;

FIGURE 9 is a fragmentary perspective view illustrating a circular-to-rectangular mode waveguide coupler, useful in connection with the apparatus lof the invention.

Referring to FIGURE 1, the instantaneous condition of a voltage wave, propagating on a uniform lossless transmission line would appear as indicated in the graphic representation wherein voltage amplitude (V) is plotted along the ordinate, and spatial position is plotted along the abscissa. This voltage wave can be simulated by a series of one-half wavelength transmission sections, graphically indicated at 1-4, which are alternately charged, as regards polarity. However, propagation of electromagnetic energy requires that electric and magnetic fields (or voltage and currents) exist simultaneously at a point in space. Current ow, and hence radiation Of electromagnetic energy can be achieved by simultaneously connecting all the half-wavelength sections together into a continuous series. Thus, if the voltage were impressed on a transmission line with characteristic impedance Z and phase velocity u, then electromagnetic waves could propagate in each direction. In the above equation k is the phase constant which is specified by the separation of the peaks of the voltage wave and is given by k=21r/fr when T=wavelength. If, however, one end of the line is shorted while the other end is terminated in its characteristic impedance at x=H, all of the energy initially stored as electrostatic energy will travel down the line into the load in a single burst of electromagnetic radiation. Power (P), pulse duration (T), and radian frequency (w) of this burst of electromagnetic radiation is given by:

since uc, where c is the velocity of light.

The geometry appropriate for the microwave region involves a circular waveguide 24 (as shown in FIGURE 3) which will propagate the TMm waveguide mode. To visualize the voltages and currents corresponding to this mode, assume circular rings of positive and negative charge spaced apart by one-half wavelength as indicated at 25 in FIGURE 3. With such geometry, it is obvious that currents will only iiow in the axial direction as indicated by arrows 26 and 27, and there will be no circumferential current ow. The ux lines of the concomitant electrical elds are indicated by arrows 28-31. The magnetic fields co-existing with the electrical fields indicated by arrows 28-31 have a circular configuration as indicated by dotted lines 32-34 which are co-axial with the major longitudinal axis of the waveguide 24. By separating the waveguide into a plurality of individual segments, each of which is one-half wavelength long, the voltages may be statically maintained along the length of the apparatus. It only remains to establish an actual current flow along the major axis of the waveguide 24 to propagate an electromagnetic wave. There is shown in FIGURE 2 a typical construction of apparatus which will accomplish this function. This embodiment comprises a central hollow tube 5 fabricated from a dielectric material and having an inside diameter which is compatible with the frequency of the RF energy to be propagated. Tube 5 is terminated at one end by conductive disc 6 and is open at the opposite end, from which the propagated energy is to 'be emitted. The open end is indicated at 7 in FIGURE 2. There is disposed along the longitudinal axis of tube 5 a plurality of conductive waveguide sections 8-15. These sections (8-15) may be fabricated from any suitable metal and are provided with a gap extending completely through each section from end to end. A typical gap is indicated at 16 in waveguide section 15. The function |of these gaps will be described hereinafter. The number of waveguide sections employed is determined by power Output considerations as will also appear hereinafter. The portion 'of waveguide 5 supporting waveguide sections 8-15 is enclosed by a dielectric envelope 17 which, for example, may be fabricated from glass. The interior of envelope 17, in the embodiment to be described lirst, is partially evacuated to provide an internal pressure which is lower than atmospheric pressure.

Helix 18 encircles envelope 17 and is energized to produce an RF field which establishes a plasma phenomena as required to effect switching between the waveguide sections 8-15. Helix 18 is connected to an RF supply (not shown), of any suitable and well known construction, via leads 19 and 20.

Alternate waveguide sections, e.g., 3, 10, 12 and 14, are connected to common lead 22. The interposed waveguide sections, eg., 9, 11, 13 and 15, are connected to common lead 23. Lead 22 is connected to the positive terminal of a source of high voltage DC power and lead 23 is connected to the negative terminal of the DC power source. The DC power source is not shown since it may be of any suitable and well known construction capable of establishing an electrostatic potential of the desired magnitude on the respective waveguide sections.

Having described the construction of one form of the invention, the operation of the above described apparatus will now be described.

The invention makes use of three basic concepts: (l) A controlled-inode plasma technique wherein essentially solid wall waveguide propagation is achieved in the TMm mode through an RF-generated plasma without loading the RF coil or influencing the plasma properties, (the letters TM refer to transverse magnetic, while the subscript 01 designates a particular microwave field configuration); (2) A self-generating plasma waveguide property of high-power free-space. TMol mode propagation in unbounded plasmas, wherein the high-power microwaves interact with the plasma to form a TMm waveguide structure by selective ionization of the plasma; and (3) The use of an electrostatically-charged artificial transmission line to radiate an electromagnetic wave. This lastmentioned concept requires that charged sections 8, 10, 12, and 14 of the transmission line be connected to the interposed, oppositely charged sections 9, 11, 13, and 15 through suitable gap switches. Closing the gap switches will allow current to ow in a manner analogous to a wave propagating on a transmission line and thus provide RF radiation.

Various means for effecting the desired switching, to connect the sections in series, may be used. The method for effecting the necessary switching, employed in the embodiment of FIGURE 2, utilizes induced plasma phenomenon. The application of high voltage pulse to helix 18 will result in the generation of a plasma within envelope 17 causing an electron path to be established between the various waveguide sections 8 15. This switching arrangement is diagrammatically illustrated in FIGURE 5 wherein section 8, which is positively charged, is effectively connected to waveguide section 9 by means of a plasma 24 formed between the two sections. Similarly, waveguide section 9 is electrically connected to waveguide section 10 by means of the plasma discharge 25 between the two sections. Sections 1G and 11 are also connected by means of plasma discharge 26, and sections 11 and 12 are similarly connected via plasma discharge 27, and so forth. Thus, the various waveguide sections are electrically connected in series in response to the application of a high voltage pulse to helix 18, thus permitting a current to flow. The simultaneous existence of electrical and magnetic fields (that is, voltage and currents) at a point in space permits this current iiow, and hence electromagnetic energy is radiated and propagates in the direction of arrows 28.

To prevent the circular flow of current in the waveguide sections (8-15), a break in the peripheral or circular path of the waveguide is provided. This is accomplished by the gap such as indicated at 16 in FlGURE 2. The

continuity of circular mode currents may be interrupted by more than one gap, although a single gap is sufficient.

To enhance microwave propagation, a modification of the apparatus as shown in FIGURE 6 may be employed. In this modification, individual waveguide sections 31-35 (as shown in FIGURE 6) cor-respond generally to waveguide sections 8 12, shown in FIGURE 2, but in this instance are provided with special choke flanges similar to those used heretofore in conventional waveguide practice. These flanges comprise an annular groove or slot along one end of each waveguide section. A typical slot is indicated at 36 in waveguide section 33. This slot 36 is one quarter wavelength in depth, thus causing the microwaves to effectively see a short circuit at the inner surface of the waveguide. That is, the generated wave will behave as if there were a solid waveguide even though the waveguide wall has gaps in it as a result of it being divided into the various sections 31-35. FIGURE 7 is an enlarged View which more clearly indicates that the depth of the groove 36 has a dimension of one-quarter wavelength and that the wall section of the waveguide section is one-quarter wavelength thick. These -choke joints also act as narrowband filters which suppress harmonics in the radiated wave.

The rapid application of RF voltage to leads 19 and 20, either by switching on the RF power supply, or by a suitable pulse generator as will be described more fully hereinafter, will ionize the gaps separating the waveguide sections and in effect, make a gas switch at the junction between every waveguide section. This increased ionization then gives rise to an enchanced DC breakdown between the waveguide sections. The combination of RF energy supplied to helix 18 and the DC energy supplied via leads 22 and 23 results in a breakdown which is combined with a self-generating plasma waveguide effectwhich has been demonstrated to readily propagate microwaves in the TMm mode under much less favorable geometry-to result in a rapid approach to solid wall waveguide propagation. Thus, the stored electrostatic energy is converted into TMm radiation down the dielectric cylinder 5 at a frequency determined by the spacing of the waveguide'sections (8-15) As mentioned hereinbefore, the individual waveguide sections each may be provided with more than one longitudinal slot. This has the advantage of dividing each section into a number of segments and thus providing more uniform circular discharge current. That is, DC discharge between two electrodes usually takes place over a single spark channel. The use of individual segments will break up each Wave guide section into several spark channels to more nearly approach circular symmetry as required by the TMm mode geometry.

For proper operation, the ionization or switching times must be shorter or at most, comp-arable to the period of the microwave oscillation. For X-band (10,000 megacycles) the period is *10 seconds. The rate of ionization (or the switching speed) depends on RF power level, diffusion, attachment, and recombination losses in the discharge as well as on the DC voltage level and the effectiveness of the self-generating plasma waveguide phenomena. The time lags in a simple DC breakdown are of the order of 10-7 to 10*9 seconds. The fact that a 2-microsecond X-band radar pulse (with a pulse repetition frequency of 539) can achieve a 10 foot long plasma waveguide at pressures of 1.5 to 2.0 torr, suggests that the selfgenerating plasma waveguide phenomena proceeds at an extremely rapid rate. That is, at these pressures the ionization clean-up in the afterglow is essentially complete before the subsequent pulse is initiated. Thus, the 10 foot plasma waveguide section must be created during essentially a single Z-microsecond pulse duration. Further, at very short gaps which correspond to large DC breakdown voltages, field emission breakdown may be the primary breakdown mechanism. Experiments with field emission at X-band frequencies have indicated that the time constants associated with field emission breakdown are well below the 10-10 second level.

There is shown in FIGURE 8 a modification of the switching gap which takes advantage of the field emission technique to improve switching efficiency. In this embodiment the interior of envelope 17 is evacuated to provide a vacuum similar to that provided in conventional electronic vacuum tube devices. One end of each waveguide section is provided with a plurality of filaments or needles, indicated generally at 37, which function as a cold cathode to emit the electrons for completing the circuit path between adjacent waveguide sections 38 and 39. The filaments (37) are fabricated to have extremely fine needle points to take advantage of the intense field that surrounds a sharply pointed conductor and so to hold the voltage necessary for field emission to a reasonable level. In a typical construction the hemispherical tip of each needle is from one micron to one-hundredth of a micron (l0-4 to 10-6 cm.) in radius. This arrangement permits higher current densities to be achieved more rapidly than could be provided by the planar faces of the structure described in connection with FIGURES 2, 4 and 6. It is necessary to limit the current density to some value which will minimize surface material evaporation and thus avoid changing the shape of the emitter. Changes in the geometry of the emitter will change the arcs current-voltage relation from one pulse to another. However, a single needle can deliver considerable power if sufficient voltage is applied. A needle with a tip radius of 6 10-4 cm. can emit a current of 140 amperes if pulsed at 300,000 volts for 10 billionths of a second. By varying the size and the number of the needles, it is possible to obtain a wide range of conductance or power levels. In a practical construction, field emission time constants of the order of 10-lo seconds may be achieved. A pulse voltage is superimposed on the charging voltage on 22 and 2.3 to provide a combined voltage level sufiicient to cause field emission.

The above-described embodiment, employing field emission switching, will operate at X-band and higher, frequencies thus extending the range 0f the invention beyond the upper limit of gas discharge switching techniques which, for practical purposes, has as an upper limit, frequencies of the order of megacycles. Since field emission time constants are so extremely short (being of the order of atomic velocities) the upper frequency limit is within the range of millimeter and sub-millimeter wavelengths.

It should be noted that the use of field emission, to accomplish inter-section switching, would obviate the helix 18 shown in the apparatus of FIGURE 2.

The emitted current from a field effect cathode increases exponentially with the applied electric field. Current densities of the order of 100 million amperes per square centimeter of emitter surface are theoretically attainable. Steady currents of 100 million amperes per square centimeter have been experimentally obtained and intermittent pulses of 10 times higher.

In a practical construction the field emitter may be made of tungsten fashioned in the shape of extremely fine needles. lt might appear almost impossible to fabricate a group of tiny needles suiciently uniform to operate in parallel, since in order to keep the emitted currents within 10% of one another the cathode geometries must be held to a tolerance of about 1%. Actually it is relatively simple to implement this structure. The needles are first sharpened by electrolytic polishing to a radius of about a millionth of a centimeter and then the tips are dulled by being heated in a vacuum. The rate of dulling depends on surface forces that vary with tip radius, so that the sharper needles dull faster and the radii of the individual needles become nearly equal. Manipulation of the ternperature and the electric field during this procedure provides further control of the shape of the tip. In practice a group of needles can be made to emit in parallel at a common voltage with between 70 and 100 percent efficiency.

There is shown in FIGURE 9 apparatus for coupling the output end of tube of the wave generator apparatus of the invention to a conventional rectangular waveguide. This comprises a circular-to-rectangular waveguide junction for transforming the TMm mode to a conventional rectangular mode transformation. As can be seen in FIG- URE 9, the end of waveguide tube 5 is connected to coupler 41 which transforms the circular-mode propagation to a rectangular mode which is then propagated in the direction of arrow 42. The rectangular waveguide may be coupled to any suitable utilization device such yas a waveguide horn for propagation ofthe generated electromagnetic space wave. Details of the waveguide structure beyond the circular-to-rectangular junction 41 are not shown, since such apparatus is well known and lies outside the scope of the present invention.

Among the advantages of the invention are that various frequency band generators can be constructed by simply altering the spacing ofthe charged rings. The combination of high power and extremely narrow pulse width provided by the present invention are well beyond the current state of the microwave pulse generator art. Applications of the invention include precise ranging, target identification, plasma diagnostics, lightweight high-power aircraft and spacecraft communications systems, spacecraft re-entry communications through the plasma sheath (high-power short duration millimeter wave pulses which propagate through the plasma before the plasma has a chance to exhibit self-shielding), and nanosecond pulse generation for microwave circuit diagnostics.

The use to which the invention is put actually forms no part of the invention, and the above enumerated uses are mentioned only to illustrate the utility of the invention. Since certain changes may be made in the abovedescribed techniques `and apparatus without departing from the scope of the invention hereinbefore described, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. An electromagnetic space wave generator apparatus, comprising:

a plurality of TMm waveguide sections each of which has an axial dimension substantially equal to one half of the wavelength of the space wave to be generated, disposed in spaced-apart relationship along a common axis;

first charging means for establishing a charge of a first polarity on every other one of said waveguide sections;

second charging means for establishing a charge of a second polarity on the remaining ones of said waveguide sections; and

switching means coupled to said waveguide sections for establishing a series conduction path between said charged sections thus generating a radio-frequency electromagnetic wave along said waveguide sections.

2. An electromagnetic space wave generator apparatus, comprising:

a plurality of substantially cylindrical TMm mode waveguide sections each of which has an axial dimension substantially equal to one half of the wavelength of the space wave to be generated, and disposed in spaced-apart relationship along a common axis;

a hermetically sealed envelope surrounding said waveguide sections;

first charging means for establishing a charge of a first polarity on every other one of said waveguide sections;

second charging means for establishing a charge of a second polarity on the remaining ones of said waveguide sections; and

switching means coupled to said waveguide sections for establishing a series conduction path between said charged sections thus generating an electromagnetic wave along said sections.

3. An electromagnetic space wave generator apparatus as dened in claim 2 wherein said switching means comprises:

a gas within said envelope; and

means for ionizing said gas to establish a conductive path between said charged waveguide sections.

4. An electromagnetic space wave generator as defined in claim 2 wherein said envelope is evacuated to provide a high vacuum and said switching means comprises:

a field emission cathode annularly disposed about one end of each adjacent waveguide section; and

means for applying a switching voltage to said cathodes suflicient to cause field emission.

5. An electromagnetic space wave generator apparatus as defined in claim 2 having:

choke joint means annularly disposed about one end of each adjacent waveguide section for enhancing the coupling therebetween when said series conduction path is established.

6. An electromagnetic space wave generator apparatus,

comprising:

a plurality of substantially cylindrical 'IMM mode waveguide sections each of which has an axial dimension substantially equal to one half of the wavelength of the space wave to be generated, disposed in spacedapart relationship along a common axis, and each section having at least one longitudinal gap extending from end-to-end thereof;

a hermetically sealed envelope surrounding said waveguide sections;

first conductor means connected in common to every other one of said waveguide sections;

second conductor means connected in common to the remaining ones of said waveguide sections;

voltage excitation means connected to said first and second conductor means for initially establishing charges of opposite polarities on alternate ones of said waveguide sections;

switching means coupled to said waveguide sections for establishing a series conduction lpath between said charged sections thus generating an electromagnetic wave which is propagated in the TMm mode along said waveguide sections; and

a waveguide extending through a wall of said envelope for transmission of said generated electromagnetic wave from said apparatus.

7. An electromagnetic space wave generator apparatus as defined in claim 6 wherein said switching means cornprises:

a low-pressure gas within said envelope; and

means for ionizing said gas to establish a conductive path between said charged waveguide sections.

8. An electromagnetic space wave generator apparatus as defined in claim 6 wherein said envelope is evacuated to provide a high vacuum and said switching means comprises:

a field emission cathode annularly disposed about one end of each adjacent waveguide section; and wherein said voltage excitation means includes:

means for applying a switching voltage to said first and second conductor means suflicient to cause field emission to occur from said cathodes.

9. An electromagnetic space wave generator apparatus as defined in claim 6 having:

choke joint means annularly disposed about one cnd of each adjacent waveguide section for enhancing the coupling therebetween when said series conduction path is established.

10. An electromagnetic space wave generator apparatus, comprising:

a plurality of substantially cylindrical TMm mode waveguide sections each of which has an axial dimension substantially equal to one halfof the wavelength of the space wave to be generated, disposed in spacedapart relationship along a common axis, and each section having at least one longitudinal gap extending from end-to-end thereof;

a low-pressure gas-filled envelope surrounding said waveguide sections;

a first conductor means connected in common to every other one of said waveguide sections;

second conductor means connected in common to the remaining ones of said waveguide sections;

voltage excitation means connected across said first and second conductor means for initially establishing charges of opposite polarities on alternate ones of said waveguide sections;

induction means for ionizing the gas in said envelope and thereby establish a series conduction path between said charged sections, whereby an electromagnetic wave is generated and propagated in the TMm mode along said waveguide sections; and

waveguide means extending through said envelope for transmission of said electromagnetic wave from said apparatus.

11. An electromagnetic space wave generator apparatus as defined in claim having:

choke joint means annularly disposed about one end of each adjacent waveguide section for enhancing the coupling therebetween when said series conduction path is established.

12. An electromagnetic space wave generator apparatus, comprising:

an elongate hollow dielectric cylinder;

a plurality of substantially cylindrical waveguide sections, each having a diameter such as to establish the TMm waveguide mode in said cylinder and each being one-half wavelength long, coaxially -disposed in spaced-apart relationship along the major axis of said cylinder, and each section having at least one longitudinal gap extending from end-to-end thereof;

a gas-lled envelope surrounding said waveguide sections and having an end wall containing an opening through which an open end of said cylinder extends in sealed relationship thereto; a conductive helix spaced apart from, and coaxially disposed along the exterior of said gas-filled en- 5 velope;

first conductor means connected in common to every other one of said waveguide sections; second conductor means connected in common to the remaining ones of said waveguide sections; voltage excitation means connected across said first and second conductor means for initially establishing charges of opposite polarities on alternate ones of said waveguide setcions; radio-frequency generator means connected to said helix for excitation thereof, whereby the gas within said envelope may be caused to ionize and establish a series conduction path between said charged sections thus generating an electromagnetic wave which is propagated in the TMm mode along said waveguide sections; means closing the other end of said cylinder for reecting said electromagnetic wave; and waveguide means coupled to said open end of said cylinder for transmission of said electromagnetic wave. 13. Apparatus as defined in claim 12 wherein said wave means comprises:

a rectangular waveguide; and having:

waveguide mode transformation means for converting the circular TMm mode of the microwave generated within said apparatus to rectangular mode propagation for transmission through said rectangular waveguide.

References Cited UNITED STATES PATENTS 2,683,216 7/1954 Wideroe 3l5-39X HERMAN KARL SAALBACH, Primary Examiner.

PAUL L. GENSLER, Examiner. 

